Extremely efficient photoisomerization of water-soluble diphenylbutadiene dendrimers

Article abstract of DOI:10.1016/j.dyepig.2011.05.002

A water-soluble diphenylbutadiene-cored poly(aryl ether) dendrimer, together with a corresponding lipophilic diphenylbutadiene dendrimer and a standard compound, (1E,3E)-1,4-bis(3,5-dimethoxyphenyl)buta-1,3-diene, were prepared and their photochemical properties were examined. The water-soluble dendrimer in aqueous solution exhibited an extremely high photoisomerization quantum yield (Φ_iso = 0.64), whereas the quantum yield of the bis(3,5-dimethoxyphenyl)butadiene reference compound in THF was much lower (Φ_iso = 0.26). The excitation spectra of the water-soluble dendrimer strongly depended on the emission wavelength and the fluorescence lifetime contained several components. These results indicate that the core unit in the water-soluble dendrimer adopts multiple conformations, one of which is distorted and can undergo photoisomerization quite easily.

Full text of DOI:10.1016/j.dyepig.2011.05.002

Dyes and Pigments 92 (2012) 802e806
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Extremely efficient photoisomerization of water-soluble
diphenylbutadiene dendrimers
,
Yousuke Miura a, Atsuya Momotake a, Yoko Kanna b, Yoshinobu Nishimura a, Tatsuo Arai a
*
a Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
b Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A water-soluble diphenylbutadiene-cored poly(aryl ether) dendrimer, together with a corresponding
lipophilic diphenylbutadiene dendrimer and a standard compound, (1E,3E)-1,4-bis(3,5-dimethoxyphenyl)
buta-1,3-diene, were prepared and their photochemical properties were examined. The water-soluble
dendrimer in aqueous solution exhibited an extremely high photoisomerization quantum yield
Received 20 December 2010
Received in revised form
20 April 2011
Accepted 3 May 2011
Available online 21 June 2011
(
Fiso ¼ 0.64), whereas the quantum yield of the bis(3,5-dimethoxyphenyl)butadiene reference compound
in THF was much lower (Fiso ¼ 0.26). The excitation spectra of the water-soluble dendrimer strongly
depended on the emission wavelength and the fluorescence lifetime contained several components. These
results indicate that the core unit in the water-soluble dendrimer adopts multiple conformations, one of
which is distorted and can undergo photoisomerization quite easily.
Keywords:
Diphenylbutadienes
Dendrimers
Photoisomerization
Fluorescence lifetime
Excitation spectra
Fluorescence spectra
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
way cis-to-trans photoisomerization [12,13]. One-way trans-to-cis
photoisomerization has also been observed in a water-soluble stil-
The excited-state behavior of arylethenes such as stilbenes is
well characterized and has become an important area of study
because of a number of practical and potential applications [1].
Among a number of deactivation pathways, photoisomerization
around a C]C double bond has been studied since the 1960s [2,3]
because it is an especially good model for the photoisomerization
of retinal in rhodopsin, accounting for both its high selectivity and
efficiency [4]. A retinal molecule that is covalently attached to an
opsin protein through a Schiff base moiety undergoes photo-
isomerization from its 11-cis to its all-trans form with a remarkable
quantum yield (Fiso ¼ 0.65 ꢀ 0.02) [5e7]. Generally, development of
chromophores in which the photoisomerization quantum yield
exceeds 50%, as seen in the rhodopsin system, has not been
straightforward with respect to isomerization around the C]C
double bond. For instance, for excited-state trans-stilbene, only one
half of the molecules isomerizes to the cis form, and the other half
reverts back to the original trans form (Fig. 1, energy diagram).
Despite this, some C]C double bonded molecules that control the
direction of photoisomerization have been reported [8e11]. For
example, we have developed styrylanthracene, which exhibits one-
bene dendrimer [14], but the photoisomerization quantum yield
was still less than 50% [15]. However, during the course of our
research on photoresponsive dendrimers, the isomerization effi-
ciency of stilbenes [14e16] or azobenzenes [17] was found to change
significantly within water-soluble dendrimers in aqueous media
compared to those in conventional organic solvents [18e20]. We
report here the first synthesis of a water-soluble diphenylbuta-
diene-cored dendrimer (Chart 1) that exhibits an extremely high
photoisomerization quantum yield (Fiso ¼ 0.64). This dendrimer
may serve as a model compound for a photoresponsive biomolecule.
2. Experimental section
2.1. Materials
2.1.1. Preparation of 1
A
solution of trans-3,5-dimethoxycinnamaldehyde (0.75 g,
3.9 mmol) in THF (5 mL) was added to a mixture of diethyl 3,5-
dimethoxybenzylphosphonate (1.15 g, 4.0 mmol) and NaH (0.3 g,
12.5 mmol) in THF (20 mL) at 0 ꢁC under nitrogen atmosphere. The
mixture was stirred for 48 h at room temperature. The reaction
mixture was quenched with water (100 mL) and extracted with
dichloromethane (100 mL ꢂ 2). The solvent was removed by
* Corresponding author. Tel.: þ81 298 53 4315; fax: þ81 298 53 6503.
E-mail address: arai@chem.tsukuba.ac.jp (T. Arai).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.002

Y. Miura et al. / Dyes and Pigments 92 (2012) 802e806
803
for 90 h. The solvent was evaporated after the resulting salts were
filtered. The residue was purified via silica gel column chroma-
tography (dichloromethane/ether ¼ 20/1), followed by GPC column
(TOSOH G2500HxL) using chloroform as an eluent, to give BD as
a pale yellow solid (70 mg, 36
(400 MHz, CDCl3):
32H), 6.83 (br s, 2H), 6.64 (d, J ¼ 2.2 Hz, 24H), 6.63 (d, J ¼ 2.0 Hz,
4H), 6.51 (br s, 2H), 6.50 (t, J ¼ 2.2 Hz, 12H), 6.47 (t, J ¼ 2.0 Hz, 2H),
5.04 (s, 32H), 4.95 (s, 24H), 3.88 (s, 48H). 13C NMR (100 MHz,
m
mol) in 20% yield. 1H NMR
8.00 (d, J ¼ 8.6 Hz, 32H), 7.43 (d, J ¼ 8.6 Hz,
d
CDCl3):
d 52.2, 69.5, 69.9, 101.7, 105.8, 106.5, 127.0, 129.7, 129.8,
129.9, 133.1, 139.4, 139.5, 141.9, 159.89, 159.94, 166.8. MALDI-TOF-
MS (m/z): [M þ Na]þ calcd for C116H102O28, 1942.66; found, 1943.57.
2.1.3. Preparation of WBD
A solution of BD (37 mg, 19 mmol) in mixed solvent (1 M KOH/
THF/MeOH ¼ 1/4/1, 12 mL) was stirred at 60 ꢁC for 8 h under
nitrogen atmosphere. The mixture was then acidified by the addi-
tion of 1 M HCl. The resulting precipitate was filtered and washed
with distilled water to give WBD as a pale yellow solid (32 mg,
Fig. 1. Potential energy diagram of trans-cis photoisomerization around C]C double
bonds.
17.5 m d 7.95 (d,
mol) in 92% yield. 1H NMR (400 MHz, DMSO-d6):
J ¼ 8.4 Hz, 16H), 7.54 (d, J ¼ 8.4 Hz, 16H), 7.11 (br s, 2H), 6.76e6.56
(br s ꢂ 4, 20H), 5.18 (s, 16H), 5.04 (s, 8H). MALDI-TOF-MS (m/z):
[M þ Na]þ calcd for C108H86O28, 1831.82; found, 1831.32.
evaporation under reduced pressure. The residue was purified via
silica gel column chromatography (hexane/ethyl acetate ¼ 4/1) and
recrystallized from hexane/ethyl acetate to give 1 as colorless
crystals (0.43 g, 1.32 mmol) in 36% yield. m.p. 118e119 ꢁC 1H NMR
2.2. Instrumental analysis
(400 MHz, CDCl3):
d
6.91 (m, 2H), 6.61 (m, 2H), 6.60 (d, J ¼ 2.2 Hz,
4H), 6.38 (t, J ¼ 2.2 Hz, 2H), 3.82 (s, 12H). 13C NMR (100 MHz,
1H and 13C NMR spectra were measured with a Bruker ARX-400
(400 MHz for 1H NMR, 100 MHz for 13C NMR) spectrometer in
solutions of CDCl3 or DMSO-d6 with tetramethylsilane as an
internal standard. Absorption and fluorescence spectra were
recorded on a Shimadzu UV-1600 spectrophotometer and a Hitachi
F-4500 fluorescence spectrometer, respectively. Fluorescence life-
times were determined with a Horiba NAES-1100 time-resolved
spectrofluorometer or a time-correlated single-photon counting
method as reported previously [22].
CDCl3): d 160.9, 139.3, 133.1, 129.6, 104.5, 100.1, 55.4. Anal. Calcd. for
C20H22O4: C, 73.60; H, 6.79; O, 19.61. Found: C, 73.33; H, 6.76; O,
19.91.
2.1.2. Preparation of BD
(1E,3E)-1,4-bis(3,5-dihydroxyphenyl)buta-1,3-diene (48 mg,
0.178 mmol) and G1-Br [21] (410 mg, 0.82 mmol) in a mixed solvent
of THF (6 mL) and DMF (2 mL) were added to a mixture of 18-
crown-6-ether (33 mg, 12.5 mmol) and K2CO3 (173 mg, 1.3 mmol)
under a nitrogen atmosphere, and the mixture was stirred at 60 ꢁC
2.3. Photoirradiation
For ultravioletevisible spectroscopic studies, 3.00 mL of sample
solution was added to a 1 cm path length quartz cuvette. The
absorbance at the peak wavelength of irradiated sample was 0.2.
Deaerated solutions were prepared by bubbling the solutions with
dry argon for 10e15 min to reach a 3.0 mL volume. The quantum
yield of photoisomerization was determined by irradiation at
325 nm from a 150 W xenon lamp through a monochrometer using
diphenylbutadiene (Ftt/tc ¼ 0.11 in cyclohexane) [23] as a stan-
dard. Note that the quantum yield of photoisomerization of WBD is
defined as an isomerization quantum yield for trans,trans form-to-
cis,trans or cis,cis form, since the product ratio (cis,trans/cis,cis) has
not been determined. In these experiments the total conversion
was always <10%.
3. Results and discussion
3.1. Photoisomerization
The photoisomerization quantum yield of WBD in aqueous
solution was unpredictably high (Fiso ¼ 0.64) in terms of the
conventional photoisomerization mechanism of C]C double bonds
as mentioned above (Table 1). Upon illumination of light at the
maximum absorption wavelength, the absorption band due to
the diphenylbutadiene core drastically decreased while a peak in
the shorter wavelength region increased (Fig. 2a). This manner of
spectral change during photoisomerization is different from that of
well-known diphenylbutadiene (DPB) in which the mutual
Chart 1. Chemical structures of G1-Br, 1 and diphenylbutadiene-cored dendrimers (BD
and WBD).

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Y. Miura et al. / Dyes and Pigments 92 (2012) 802e806
Table 1
Photochemical properties of 1 and WBD.
labs/nm
338
275, 335
3
/Mꢃ1 cmꢃ1
51500
16900, 28600
s
s/ns
1.2
0.15, 1.6, 9.1
Fiso
1
0.26
0.64
WBD
isomerization takes place between the trans,trans (tt) form and the
cis,trans (ct) form [23].Photoisomerization of WBD may produce
the cis,cis (cc) form along with the ct form (Scheme 1).In contrast,
the spectral change during the photoisomerization of 1 in THF
(Fig. 2b), with a quantum yield of Fiso ¼ 0.26, is similar to that of
DPB. This spectral difference between WBD and 1 probably origi-
nates from the difference in the ground state conformation, which
is supported by the difference in the absorption spectra before
irradiation. The band for the core of WBD is broadened and less
intense compared to that of 1. The extinction coefficient of
WBD was determined at the maximum wavelength to be
Scheme 1. Photoisomerization in WBD.
curve. On the other hand, the fluorescence decay curve of WBD was
fitted to a three-component exponential decay (Fig. 3), likely due to
the diphenylbutadiene core in water-soluble dendrimers adopting
multiple conformations in the ground state, thus producing
a multi-emission state. The estimated lifetime is 1.2 ns for 1 and
0.15, 1.6, and 9.1 ns for WBD (Table 1). The lifetime of 1.6 ns is
assigned to the planar conformation, and the shorter component of
0.15 ns may be attributed to the distorted conformer [24]. The
longer lifetime (9.1 ns) component is assigned to the s-cis form,
which has been reported to have a longer lifetime than that of the
planar conformation [25].
28,600 Mꢃ1 cmꢃ1, which is almost half that of 1 (51,500 Mꢃ1 cmꢃ1
,
Table 1). In addition, the maximum wavelength of WBD was 3 nm
shorter than that of 1. These results suggest the shrinking of WBD in
water due to the hydrophobic interactions of the dendrimer inte-
rior, which induces a twisted or distorted state of the diphenylbu-
tadiene core around the C]C double bond or a different
pep
dihedral angle.
3.3. Fluorescence excitation spectra
3.2. Fluorescence lifetime
As expected from the fluorescence lifetime analysis, the fluo-
rescence (Fig. 4a) and the fluorescence excitation (Fig. 4b) spectra of
WBD are largely dependent on the monitoring wavelength, indi-
cating that the diphenylbutadiene core in WBD adopts multiple
conformations in the ground state. A structured excitation spec-
trum was observed when monitored around 400 nm, whereas
a broadened excitation spectrum was observed with monitoring at
wavelengths longer than 450 nm. The structured excitation bands
could be assigned to diphenylbutadiene with a planar conforma-
tion because the spectral shape is similar to the absorption band of
1. The broadened spectra likely originated from the non-planar
(distorted) conformation. The wavelength dependence of the
excitation spectra indicates that the absorption spectrum of WBD
represents the sum of the bands for multiple components in the
ground state. Unlike WBD, the excitation spectrum of 1 is similar to
the absorption spectrum (Fig. 4c) and did not depend on the
Compound 1 should adopt a single planar conformation in THF.
This is supported by the analysis of the single-component decay
monitoring wavelength, which is consistent with
a single-
component fit in the fluorescence decay analysis.
Fig. 2. Change in the absorption spectra of WBD with irradiation at 334 nm in 2 mM
Fig. 3. Fluorescence decay curve of WBD in 2 mM KOH aqueous solution with exci-
KOH aqueous solution (a) and 1 with irradiation at 338 nm in THF (b) under Ar.
tation wavelength at 375 nm.

Y. Miura et al. / Dyes and Pigments 92 (2012) 802e806
805
4. Conclusions
In conclusion, the diphenylbutadiene-cored poly(aryl ether)
dendrimer WBD undergoes photoisomerization with an extremely
high quantum yield in aqueous solution. Fluorescence lifetime
analysis and excitation spectra suggest that the core diphenylbu-
tadiene adopts multiple conformations within the dendrimer and
one of the conformers with a distorted structure may isomerize
very efficiently. This is the highest isomerization quantum yield
of diphenylbutadienes reported to date and also the highest
isomerization quantum yield of a dendrimer. These findings using
a water-soluble dendrimer system may be useful for controlling
photoisomerization efficiency and developing new models of
photoresponsive biomolecules.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism” (No.
471) from the Ministry of Education, Culture, Sports, Science, and
Technology (MEXT), Japan.
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